The aim of the proposed research is to understand the molecular arrangement of the ribsomal genes and understand the natural mechanisms whereby an organism can correct ribosomal gene deficiencies. We have isolated from Drosophila melanogaster DNA of estimated molecular weight 5 x 10 to the 9th power daltons and have demonstrated the existence of slower sedimenting ribosomal gene which we have tentatively considered not to be integrated into the DNA of the chromosome. In addition to occurring in the polytene salivary glands of larvae, they occur in adults and the predominantly diploid larval brains and imaginal discs of certain genetypes heterozygous for rearrangements having breakpoints in the proximal heterochromatin between the nucleolus organizer and the centromere. The proposed research will probe the structure of these genes in different genotype using sucrose sedimentation, ethidium bromide, partial DNase digestion, and electron microscopy. Similar studies will probe the structure of the 5S genes and satellite sequences. The possibility that unintegrated genes are involved in unstable changes in ribosomal gene reiteration will be investigated. In addition, the structure and replication of the ribosomal genes in cleaving embryos will be investigated. These results are expected to have implications for models of chromosome structure, replication, and the pairing of homologous chromosomes. Health problems to which the research related include genetic diseases and cancer.